This invention relates, in general, to sound transmitting devices and, in particular, to a new and useful protective breathing mask which has both a speaking diaphragm for close communication and an electroacoustic transducer system for indirect speech transmission from inside the mask.
The invention relates particularly to a protective breathing mask which in addition to having a speaking diaphragm for close communication is also equipped with an electroacoustic transducer system for indirect speech transmission from inside the mask.
The prior art provides a number of methods of speech transmission for use with a protective breathing mask. A typical prior art system of this kind is described in German Unexamined Patent Disclosure (Offenlegungsschrift) No. 30 13 939, which relates to a microphone-loudspeaker to be used selectively with a protective breathing mask or safety helmet. The microphone-loudspeaker, consisting of an electroacoustic, preferably dynamic transducer, fits into a cup-shaped housing that is removably attached by its cylindrical rim to the rim head of the exhale valve housing of the breathing mask or to the chin protector of the safety helmet. The sole advantage of such a transducer system is that one and the same transducer can be used both as a microphone and as a loudspeaker. A microphone-loudspeaker, however, always suffers from disadvantage of reduced transmission quality for acoustic reasons. Another disadvantage is that the microphone can only pick up the sound spoken through the mask, which is considerably impaired in intelligibility. This makes it even more likely that sound will be picked up from the close vicinity of the person wearing the protective breathing mask, and what is primarily transmitted is interference sound containing no information, such as noise from the exhale valve.
In order to achieve a tolerably useful degree of speech intelligibility, the antechamber of the exhale valve must be designed as a Helmholtz resonator, with a resonance frequency of around 2,400 Hz, and another Helmholtz resonator with a resonance frequency of 3,000 Hz must be built into the microphone-loudspeaker itself in order to guarantee that the frequency range to be transmitted for radiophone communication is at least 3,000 Hz.
The low and unsatisfactory reproduction quality of microphone-loudspeakers is due to the fact that vibration conditions for the transducer diaphragm are different for sound reception than they are for sound reproduction, and electrical means are required to harmonize the frequency patterns of the two transducers. It should also be noted that a diaphragm diameter of 3 to 4 cm, such as is customary in such microphone-loudspeakers, is not adequate for the reproduction of the lower frequencies. The nonlinear distortion factor is also correspondingly high in any such small loudspeaker systems. It must be considered, therefore, whether unmistakeable speech intelligibility is assured in emergency use by a microphone-loudspeaker of the type described, since a misunderstanding could have fatal consequences.
In German Unexamined Patent Disclosure No. 30 13 939, various prior art devices for person-to-person communication are described. Thus, walkie-talkies, compact phone sets, necklace microphones, microphone systems to be carried on the cheekbone under the ear and microphone attached to the exhale valve of a protective breathing mask are proposed for use, all of which involve some sort of disadvantage. For instance, to operate the walkie-talkie device, the wearer must have one hand free, which in use considerably hinders the wearer in his freedom of movement. In another example, the arm of the phone set bearing the microphone must be swiveled around to the mouth area after the gas mask has been donned, and at that spot only a largely unintelligible speech signal spoken through the mask can be picked up. The other systems mentioned are mostly inadequate from an acoustical standpoint and are also uncomfortable to carry, or else the influence of interference sounds such as surrounding noise and valve noise from the protective breathing mask impairs the quality of the speech signal.
German Unexamined Patent Disclosure No. 31 37 113 does, it is true, mention a helmet/breathing mask arrangement that is equipped with a contact microphone, but means by that a microphone of the kind zlready described in German Unex. Patent Disclosure No. 30 13 939, which picks up speech vibrations directly at the head.
AT Patent No. 342,129 also discloses a gas or smoke mask that has a microphone in the cheek piece in the area near the mouth on the inside of the shell consitituing the gas mask that directly picks up the air sound waves created by speech. Since this mask is not equipped with a speaking diaphragm, even for close communication the built-in microphone must be used, which can sometimes come into direct contact with the cheek of the wearer of the gas mask, whereupon intelligibility is sharply reduced, quite apart from the disadvantageous position of the microphone to the side of the mouth, a position in which the higher frequencies, which contribute largely to perfect intelligibility, are greatly weakened.
German Unexamined Patent Disclosure No. 3 127 677 discloses a speaking device for mask wearers which entails at the minimum setting up a a transmission arrangement on the outside of the mask that is capable of producing an output signal corresponding to the voice of the mask wearer. The output signal can then be conveyed to a loudspeaker that the mask wearer is carrying on his person in order to produce sound signals that can be heard by persons in the vicinity of the mask wearer. It is clear that this prior art arrangement does not include a speaking diaphragm for close communication, but instead requires a complicated electroacoustic system in order to accomplish it. Furthermore, in the prior art arrangement the sound must penetrate the mask material, which does not make for clear intelligibility outside the mask of what is spoken. Moreover, the design expense is considerable and is all out of proportion to the results obtained.
Finally, in German Patent Publication (Auslegeschrift) No. 708 045, a mask connecting piece is described with at least one exhale valve and a speaking diaphragm. The latter is solely for close communication and involves an inner mask that both prevents the outgoing breath from going directly to the view window and conducts speech sounds to the speaking diaphragm, so that the sound waves pass to the outside via an antechamber. Under this prior art arrangement, there is no possibility of transmitting over a distance what is spoken inside the mask.